Prostate cancer is a major cause of cancer morbidity and mortality of older VA patients. Androgen ablation therapy is initially successful, but relapses occur due to the development of castration resistant prostate cancer (CRPC). Currently treatment options for CRPC are limited, indicating a need for the identification of new therapeutic targets. Studies indicate that the E2F3 transcription factor is overexpressed in prostate cancers, and overexpression correlates with a poor clinical outcome. The E2F3 locus encodes two isoforms, E2F3a and E2F3b, but, due to technical reasons, previously reported studies only examined E2F3a expression. Preliminary data from our laboratory indicates that prostate tumor derived cell lines express higher levels of E2F3a and E2F3b than non-transformed prostate cells. A comparison of human prostate tumors and control benign prostate hypertrophy samples indicates some tumors have elevated E2F3a and b. siRNA mediated decrease of E2F3b in tumor derived cells has a more severe effect on cell physiology than a decrease of E2F3a, indicating that E2F3b has a unique role in tumorigenesis. Additionally we found that E2F3b regulates the expression of the microRNA processing nuclease Dicer. Previous studies reported that Dicer expression is elevated in advanced and metastatic prostate tumors. Our studies link the E2F/RB signaling pathway to the miRNA machinery. We hypothesize that E2F3b promotes prostate tumorigenesis by increasing proliferation, and altering adhesion and motility. The proposed studies have three specific aims. In Specific Aim 1 we will define the role of E2F3b in proliferation, cell adhesion and motility using prostate tumor-derived cell culture lines. Additionally, the role of E2F3b in the regulation of the endogenous Dicer gene will be characterized, and lastly, the role of E2F3 isoforms in regulating miRNA clusters will be examined. Studies proposed in Specific Aim 2 will focus on genetically engineered mouse models. The expression of the E2F3 isoforms and of Dicer will be studied in three models of prostate cancer: 1) SV40 T-Antigen, 2) high levels of c-myc, and 3) Nkx3.1 and Pten tumor suppressor hemizygosity. These models mimic pathway alteration identified in human tumors. The studies in Specific Aim 3 will characterize E2F3 isoform expression in human tumor samples, in surrounding tissue and BPH control samples using immunohistochemical, Western immunoblot, and quantitative RT-PCR analysis of micro-dissected tumor tissue. Complementary studies will characterize the expression of Dicer in tumor and non-tumor tissue. Lastly, the expression of the E2F3 isoforms and Dicer will be correlated with disease progression and response to therapy. The studies will determine if E2F3b or E2F3b targets are potential therapeutic targets to halt prostate tumor progression.